Advanced ceramics have promise in a wide variety of high technology and high temperature applications. Due to their high cost, advanced ceramics have been used primarily in the aerospace industry and for military applications. Although substantial growth in the market for advanced ceramics and advanced ceramic composites has been predicted for some years, the expected growth has not occurred at least in part due to the high cost associated with producing and fabricating such advanced ceramics. Advanced ceramics and composites simply cannot compete with cheaper metals or polymers in many applications.
Traditional powder based methods for manufacturing advanced ceramics and ceramic composites have many inherent difficulties. One difficulty is the need to burn off organic binders and to sinter the ceramic products by heating suitable starting materials to extremely high temperatures for long periods of time. Sintering is required for most ceramics to impart the desired physical and structural properties. Covalent ceramics like silicon nitride (Si.sub.3 N.sub.4), are inherently difficult to sinter due to their low self-diffusivities. Consequently, even longer periods of heating and/or higher temperatures are needed to sinter the covalent materials properly. Unfortunately, voids are created during heating and sintering and often remain in the finished ceramic products. The voids have deleterious effects on the physical and structural properties of the ceramic products. Sintering acids may be used to reduce the formation of voids. However, sintering acids have deleterious effects on the high temperature strength of the resulting ceramics. Chemical-based methods have been developed in an attempt to address and circumvent some of these difficulties.
The major objectives of chemical-based methods are to reduce voids created during burn-off of organic binders and to improve ceramic sintering results. Examples of chemical-based methods are `sol-gel` and `preceramic polymer` techniques, such as those described by K. J. Wynne and R. W. Rice, Ann. Rev. Mat. Sci., 14, 297 (1984). The `sol-gel` technique primarily is applied to oxide glasses and ceramics such as silica, titania, cordierite, etc. The `preceramic polymer` technique generally is applied to non-oxide ceramic products such as silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), boron nitride (BN), etc.
Even chemical-based methods still require the starting materials and/or the intermediates to be heated, using conventional methods, to extremely high temperatures, sometimes well over 1200.degree. C., for long periods of time. The energy efficiency of the conventional heating methods is low and the cycle time is long. The starting materials and/or the intermediates also are exposed to constantly changing temperatures during the heat-up period until the desired temperature is reached and maintained.
In order to make advanced ceramics and ceramic composites, particularly silicon nitride based non-oxide ceramics and ceramic composites, that are more uniform in quality and more competitive in cost, a need exists to develop methods to manufacture and fabricate ceramic or ceramic composite materials at lower temperatures, in shorter periods of time, and with faster, more uniform heating techniques.